In current Code Division Multiple Access (CDMA) communication systems, a Radio Link Protocol (RLP) is utilized for the link layer to transport data traffic between a mobile unit and infrastructure equipment. RLP is a Negative-Acknowledgment (NAK) based protocol in that the receiver does not acknowledge correctly-received RLP frames. In-order delivery is accomplished with the use of a sequence number (SEQ) on each frame. RLP maintains a counter for the sequence number of the next new data frame to send [SEQ(S)] and a counter for the sequence number of the next new data frame it expects to receive [SEQ(R)]. RLP requests the retransmission of RLP frames when a frame is received with a sequence number greater than the next expected sequence number (SEQ(R)). Retransmission is accomplished by sending a NAK to the transmitter identifying the sequence number of the frame not received. Prior to receiving the NAK'd frame, subsequently transmitted RLP frames continue to be received by the receiver.
FIG. 1 illustrates the current RLP NAK procedure. As shown, frames 101 are transmitted by a transmitter over the air and are received by a receiver as frames 102. During over-the-air transmission, oftentimes data is lost and needs to be retransmitted to the receiver. This is illustrated in FIG. 1 as frame F3 being lost. Upon receiving frame F4, the receiver immediately realizes that frame F3 is missing and requests retransmission of F3 by sending a NAK to the transmitter. The current RLP protocol standard specifies that the NAK control frame takes precedence over any RLP frames carrying data or other channel information. Upon reception of the NAK, the transmitter retransmits F3.
It should be noted that a single NAK can request retransmission of several RLP frames (as shown in FIG. 2). In FIG. 2 RLP frames F2 and F3 are lost. Upon reception of the RLP frame F4 the receiver detect the sequence gap and sends a single NAK frame requesting retransmission of 2 RLP frames, F2 and F3.
A problem arises when the receiver fails to receive multiple frames in a short period of time. In that situation a steady stream of NAK frames will be transmitted by the receiver, congesting the communication link from the receiver to the transmitter. An example of this problem is shown in FIG. 3. In a typical application (e.g., web browsing), data flow is asymmetrical. For large Frame Erasure Rate (FER) or for cases where many sequence numbered frames arrive per frame interval, there is a high probability of having at least one RLP frame being erased in each transmission interval. In FIG. 3, RLP frames F4, F7, F15, and F18 were erased in 4 consecutive transmission intervals. The gaps in sequence numbers will generate 4 consecutive NAK frames requesting retransmission of the missing RLP frames. No other user/control data (such as TCP/IP ACKs) can be transmitted from the receiver to the transmitter while this situation persists. Because the client needs to respond to each packet downloaded by sending a TCP/IP ACK, the TCP/IP ACKs are delayed while waiting for an interval where no RLP NAKs are being sent.
Prior-art methods have attempted to deal with the flood of NAK frames that result during poor channel conditions. For example, U.S. Pat. No. 6,112,323 entitled “METHOD AND COMPUTER PROGRAM PRODUCT FOR EFFICIENTLY AND RELIABLY SENDING SMALL DATA MESSAGES FROM A SENDING SYSTEM TO A LARGE NUMBER OF RECEIVING SYSTEMS” describes a system of NAK suppression where a delay time is selected according to a defined probability function. The above technique results in a predictable number of NAK frames being sent after a given delay time. By adjusting the probability density as a function of various system parameters (such as the network packet loss rate), the number of NAK frames sent at a given time can be adjusted to suit network conditions.
Although the above-procedure does reduce the impact of multiple NAKs being sent, the above-procedure still results in NAKs being sent at inopportune time periods. Therefore, a need exists for a method and apparatus for NAK suppression within a communication system that more accurately determines the best time period to transmit NAK frames in order to reduce system resource load.